Radio Shack TRS-80 Owner's Manual page 168

Li
RS-80
MODEL
111
DISK
SYSTEM
Random
Access Techniques
Random
access offers several
advantages over
sequential access:
•
Instead of
having
to start
reading
at the
beginning of a
file,
you can
read
any
record
you
specify.
8
To update
a
file,
you
don't
have
to
read
in
the entire
file,
update
the data,
and
write
it
out again.
You
can rewrite or
add
to
any
record
you
choose, without
having
to
go through any of
the other records.
8
Random
access
is
more
efficient
—
data takes
up
less
space and
is
read
and
written
faster.
"
Opening
a
file
for direct
access allows
you
to
write
and
read
from
the
file
via
the
same
buffer.
•
Random
access provides
many
powerful statements and
functions to structure
your
data.
Once you have
set
up
the
structure, direct input/output
becomes
quite simple.
The
last
advantage
listed
above
is
also the
"hard part" of
direct access.
It
takes
a
little
extra thought.
For
the
purposes of
direct access,
you
can think of a disk
file
as a set
of
boxes
—
like
a wall of post-office boxes. Just
like
the post office receptacles, the
file
boxes
are
numbered.
We
call
these
boxes
"records."
You
can
place data
in
any
record, or read the contents of
any
record, with
statements
like:
PUT
1
»5
write buffer-
1
contents
to
record
5
GET
1
»
5
read the contents of record 5 into buffer-
In
Figure
14,
we
assume
a record length of 256.
#6
'256
iBYTES,
#1
(256
)
\bytes/
#7
){l
56
'
erresy
#2
(lAts)
#8
(258
\
\
bytes;
#3
#9
\B
7
"'
m
(256
)
VBYTES/
#10
mi
RECORDS
IN
DISK
FILE
Figure
14.
get and
put.
I/O
BUFFERS
IN
RAM
150